The present invention relates generally to internal combustion engines and, in particular, to a method for controlling operation of intake or exhaust valves independent of a fixed timing mechanism to provide flexible regulation of engine gas flow.
There are three types of gas flow in an internal combustion engine: (1) the intake fresh air flow from intake port into cylinder; (2) the gas flow from cylinder into intake port and/or exhaust port; (3) the reverse exhaust gas flow from exhaust port and/or intake port into cylinder. These engine gas flows are controlled by valve timing and valve lift. All engine performance parameters (e.g., torque, emissions, fuel consumption) are directly affected by the gas flows. But the three types of gas flow have different roles or different weight of importance for a particular engine performance parameter. For example, for air-to-fuel ratio and engine power the intake fresh air flow has a major influence. For turbocharger speed the exhaust flow has a major influence. For the emissions level of NOx the internal EGR flow has a dominant influence.
It is well known that variable valve timing can address the trade-off in engine breathing characteristics restricted by a conventional camshaft. Variable valve timing allows valve timing to be optimized, hence the engine gas flow to be regulated for each firing operating condition to achieve the best engine performance (torque, emissions, fuel economy, etc).
Emissions is one of the most important aspects in engine performance. Engine gas flow is directly related to an important emissions-control technique, exhaust gas recirculation (EGR). It is well known that EGR is an effective technique for reducing NOx (nitrogen oxygen) emissions in internal combustion engines. Generally an EGR system causes some portion of the exhaust gases to be reintroduced into one or more combustion chambers. Typically, an external EGR system is used. External EGR flow is a special type of gas flow inducted from exhaust manifold into intake manifold through external tubing when back pressure (i.e., pressure at exhaust manifold) is higher than boost pressure (i.e., pressure at intake manifold). A majority of all EGR systems in production today are external EGR systems. An advantage of external EGR is that the recirculated exhaust gas can be cooled down before it flows back to the intake manifold and the combustion chambers. With cooled external EGR, engine volumetric efficiency is higher. However, cooled external EGR systems suffer from the accumulation of soot deposits in elements of the EGR system, such as the EGR cooler. This can cause deterioration in EGR flow efficiency over time and, hence, overall engine performance.
In an internal EGR system, the exhaust gas is kept inside the combustion chamber by inducting exhaust flow back into the cylinder from exhaust port (or manifold) and/or intake port (or manifold) without external tubing, i.e., through one or more engine valves. Therefore, internal EGR flow belongs to the above-mentioned third (3) gas flow type. Variable valve timing can provide optimized internal EGR level to achieve better emissions without the need of an external EGR system. The concept of internal EGR is further illustrated with reference to FIG. 1. FIG. 1 illustrates a graph of valve lift profiles (i.e., the level to which a valve is open) as a function of crankshaft angle for a typical 4-stroke engine. In particular, FIG. 1 illustrates an exhaust valve lift profile 102 and an intake valve lift profile 104. The four strokes are typically 0xc2x0-180xc2x0 as expansion stroke; 180xc2x0-360xc2x0 as exhaust stroke; 360xc2x0-540xc2x0 as intake stroke; 540xc2x0-720xc2x0(or 0xc2x0) as compression stroke. These ranges may vary depending on the particular engine application. Usually, there exists an interval 106 centered around the transition between the exhaust stroke and the intake stroke, known as valve overlap, where both the intake valve and the exhaust valve are simultaneously open. During this interval 106, internal EGR occurs because the intake stroke causes some exhaust gases to be inducted back into the combustion chamber.
However, as known in the art, the operation of exhaust valves and intake valves is typically controlled through the mechanical profile of a fixed timing mechanism, e.g., a rotating camshaft. Because the geometry of the camshaft is fixed, the timing of the intake and exhaust valves is likewise fixed. As a result, the duration of any valve overlap as a function of crankshaft angle is also fixed. However, as the speed of engine increases, the duration of valve overlap as a function of time decreases, thereby providing less internal EGR. It is thus very difficult to satisfy the EGR demand at every engine condition when valve overlap is fixed as shown in FIG. 1.
Due to the pulsating nature of exhaust flow, it is feasible to increase the amount of internal EGR by holding the engine valves open at certain desirable periods outside of the scope of the mechanical cam profile when a favorable pressure difference exists. Thus, it would be advantageous to provide a system in which high levels of internal EGR flow rate may be achieved in an economic and effective fashion through variable valve actuation that is independent of the fixed timing mechanism.
As known in the art, another important function which can be achieved by using variable valve timing is engine braking. Most medium-duty and heavy-duty diesel engines today are equipped with a compression brake device comprising one or more valve actuators coupled to one or more exhaust valves. However, such devices are intended to be used only during engine braking but not in other operating conditions. As such, compression brake systems remain idle during most regular engine operation. One feature of compression brake devices is that its maximum stroke (or exhaust valve lift) is limited (for example, 2 mm) in order to prevent contact between the exhaust valve and piston at all operating conditions. Moreover, most compression brake devices have electro-hydraulic control features and are therefore flexible in timing control. These features are sometimes referred to as limited-stroke variable valve actuation (LSVVA).
Another trend in the automotive industry today is to develop a fully flexible air valve system or camless system. Camless system will bring tremendous advantages but are very complex and presently require significant development time and cost. Rather than switching directly to camless systems, it would be advantageous to enhance existing cam-driven valve systems with a certain degree of flexible control on opening/closing timing and valve duration. In this manner, engine manufacturers could fully utilize existing technologies to achieve the best performance with the least cost and complexity.
The present invention provides a technique to enhance existing fixed timing valve systems with flexible and independent control of the timing and lift of engine valves. To this end, the present invention is characterized by a mixed valve event combining the fixed timing profile with a variable profile produced by a valve actuator coupled to one or more engine valves. In a presently preferred embodiment, the valve actuator comprises a pressure-actuated piston coupled through a controllable fluid control valve to a source of pressurized hydraulic fluid. In this manner, the present invention facilitates the flexible regulation of engine intake and exhaust flow at different engine operating conditions. For example, the amount of internal EGR flow can be directly controlled and modulated, or engine braking can be applied as needed in vehicle operation. Additionally, operation of a turbocharger may be used, along with the desirable control provided by the valve actuator, for the purpose of controlling any of intake air flow, exhaust gas flow and/or internal EGR flow.
The following drawings and description set forth additional advantages and benefits of the invention. More advantages and benefits will be obvious from the description and may be learned by practice of the invention.